DEVELOPMENT OF RF MEMS SYSTEMS

Transcription

1 DEVELOPMENT OF RF MEMS SYSTEMS Ivan Puchades, Ph.D. Research Assistant Professor Electrical and Microelectronic Engineering Kate Gleason College of Engineering Rochester Institute of Technology 82 Lomb Memorial Dr. Rochester, NY tel: fax:

2 Outline What are RF MEMS? Basic Receiver Architecture Roles and advantages of RF MEMS Devices RF MEMS Devices and Applications RF Switches RF Resonators Integrated Process Flows Challenges Summary 2

3 What are RF MEMS? MEMS Micro Electro Mechanical Systems RF Radio Frequency applications C. T.-C. Nguyen and R. T. Howe, An integrated CMOS Micromechanical resonator high-q oscillator, IEEE J. Solid-State Circuits, vol. 34, no. 4, pp , April Ohmic switch and relays DC switching Capacitive switch Tunable RF Circuits Capacitive relay Matching networks Mechanical resonator Band pass filters Bulk acoustical resonator Local Oscillators 3

4 Basic Receiver Architecture Superheterodyne receiver Uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency. Demodulator or detector Initial tuning can be done manually, variable cap. LO tracks tuning. Mixing of incoming f d and local f LO Results in f LO -f d, f LO +f d, f d, f LO and harmonics. f LO tracks incoming f d so that f IF is always the same Bandpass filter f IF =f LO- f d f IF is constant IF 455 khz fo AM radio 10.7 MHz for broadcast FM receivers, 38.9 MHz (Europe) or 45 MHz (US) for television 70 MHz for satellite and terrestrial microwave equipment. 4

5 RF Tuning, Mixers and Band-Pass Filter Circuits Band-pass Filters Non-linearity Mixers Sampling mixers 5

6 Passive RF Components Typical passive devices: LC Resonators Bulky/large inductors and capacitors Low Q (low frequency selectivity, lossy Energy loss) Passive Devices with high Q s: Ceramic RF Filters SAW RF Filter surface acoustic wave filter Quartz Crystal Off chip inductors Disadvantages Off chip, large, expensive RF MEMS: Variable capacitors and switches Mechanically resonating structures Advantages: high Q, inexpensive, small and potentially integrated 6

7 RF MEMS Switches, switched capacitors and varactors Ohmic switch and relays Tunable RF Circuits Matching networks Deflecting cantilever or fixed-fixed beam Electrostatic, electrothermal, magnetostatic or piezoelectric Lateral or vertical Series or shunt Capacitive or ohmic RF Switches a) Capacitive shunt, fixed-fixed beam b) Ohmic series cantilever beam "Modeling RF MEMS Devices", by K. Van Caekenberghe, IEEE Microwave Magazine, vol. 13, no. 1, pp , Jan.-Feb

8 Ohmic RF MEMS Switches 8

9 Vibrating RF MEMS Resonators Filters and resonators Vibrating beam, comb, disc or ring which is sufficiently isolated from the surroundings in order to obtain a high Q m. Actuation mechanism Electrostatic, piezoelectric, thermal Suspension Fixed-fixed, free-free, stem Vibrating geometry Beam, comb, disc, ring Vibration mode Bulk (extensional), elliptical (wine glass), flexural, radial contour, torsional C. T.-C. Nguyen and R. T. Howe, An integrated CMOS Micromechanical resonator high-q oscillator, IEEE J. Solid-State Circuits, vol. 34, no. 4, pp , April

10 Types by vibration mode Vibration mode Bulk (extensional), elliptical (wine glass), flexural, radial contour, torsional Higher order vibration modes cannot also be used. "Modeling RF MEMS Devices", by K. Van Caekenberghe, IEEE Microwave Magazine, vol. 13, no. 1, pp , Jan.-Feb

11 Co-fabrication of Electrostatic MEMS Switches and Resonators Three poly layers (electrode, resonators and anchor structures) Conformal sacrificial layer for the formation of small gap <100nm High Conductivity layer for low resistance switch 6 mask layers Isolated conductive material Gate <100nm gap 1um Poly Resonator 3500 Å Si 3 N 4 2 um SiO 2 N+ Doping Source Body Drain RF Electrode Resonator RF Electrode

12 Tunable Filters Switches Filters Co-fabrication of switches and resonators 12

13 RF MEMS Challenges Bandwidth, insertion loss and isolation: High R ON C OFF or low C ON /C OFF Power handling: Electromigration (JRMS) and dielectric breakdown (VRMS) limits. Reliability: Temperature drift Dielectric charging Humidity-induced stiction Fatigue Contact degradation Creep 13

14 RF MEMS Challenges Temperature drift W. -T. Hsu and C. T. -C. Nguyen, Stiffness-compensated temperatureinsensitive micromechanical resonators, Tech. Digest, 2002 IEEE Int. Micro Electro Mechanical Systems Conf., Las Vegas, Nevada, Jan , 2002, pp Solutions: Temp compensation Passive SiO2 coating to cope with young s modulus temp dependence, degenerated doping and geometry changes Active Micro-oven to control temp, bias compensation and phase locking. 14

15 RF MEMS Challenges Dielectric charging doc.utwente.nl/66583/1/rodolf_icmts_ pdf Solutions: Reduce dc bias, use ac bias, Avoid hard contact (dimples, holes) Non-dielectric switches R.W. Herfst, Characterization of dielectric charging in RF MEMS capacitive switches, Semiconductor Components, University of Twente. 15

16 Humidity-induced stiction RF MEMS Challenges Vertical/horizontal Solutions: Supercritical drying (liquid CO2 at critical point) Dry etching Hydrophobic coatings Recoverable 16

17 Contact degradation RF MEMS Challenges Solutions: Reduce bias current Avoid hard contact (dimples, holes) Vacuum encapsulation Degradation resistant materials Dickrell, D.J.; Dugger, M.T.;, "Electrical Contact Resistance Degradation of a Hot-Switched Simulated Metal MEMS Contact," Components and Packaging Technologies, IEEE Transactions on, vol.30, no.1, pp.75-80, March

18 Conclusion RF MEMS applications o Band selective receivers RF MEMS fabrication o Co-fabrication of switches and resonators RF MEMS challenges o Fabrication, materials and reliability 18

19 Appendix 19

20 RF MEMS Devices/applications RF MEMS switches, switched capacitors and varactors Ohmic switch and relays Capacitive switch Tunable RF Circuits Capacitive relay Matching networks Vibrating RF MEMS resonators Band pass filters Mixers Reference oscillators 20

21 Resonator Modeling Flexural mode fixed-fixed beam Modeled by two capacitors in series. Most resonators can be modeled this way "Modeling RF MEMS Devices", by K. Van Caekenberghe, IEEE Microwave Magazine, vol. 13, no. 1, pp , Jan.-Feb